The present invention relates to light water nuclear fuel assemblies, and more particularly, to nuclear fuel assembly grids which carry structure for deflecting coolant flow.
Nuclear reactor engineers have for many years recognized the advantage of thoroughly mixing the reactor coolant within and between flow channels associated with each fuel rod in a light water nuclear reactor fuel assembly. It is a simple matter to provide flow deflecting structure on the fuel assembly grids, but a number of other considerations must be taken into account in order to arrive at a satisfactory design. These other considerations include the increased pressure drop resulting from the added flow resistance due to the presence of deflector structure, and the added parasitic neutron absorption by the deflector structure, which can affect achievable power levels, cycle burnup, and local power distribution.
U.S. Pat. No. 4,879,090, "Split-Vaned Nuclear Fuel Assembly Grid," issued November 7, 1989, discloses a refinement to a basic deflector concept which has been known for decades. The basic concept is to provide vanes or tabs that are integrally formed on the upper edges of the interlaced strips of a grid, and bent over into the flow channel so as to deflect the upwardly flowing coolant. In the '090 patent, each channel is shown as having two or three vanes associated therewith. The vanes are initially formed in pairs with a slit and recess between them, such that each member of the pair can be bent in an opposite direction. A weld nugget is formed in the recess between the vanes, for joining the strips which intersect each other through the slit between the vanes.
One disadvantage of this type of flow deflecting vane, is that the cantilevered orientation permits considerable leakage laterally, thereby reducing the amount of flow that can be deflected in the intended direction. Secondly, during reconstitution of fuel assemblies, such vanes are easily deformed by a misaligned fuel rod, necessitating repair or fabrication delay.
Another relatively old concept is described in U.S. Pat. Nos. 3,814,666, "Staggered Cone Deflectors," issued Jun. 4, 1974; 3,844,888, "Helical Flow Deflector Cone For Fuel Element Assemblies," issued Oct. 29, 1974; and 3,663,367, "Flow Deflector For Nuclear Fuel Element Assemblies," issued May 16, 1972 and reissued Mar. 11, 1975 as Re.28,362. The concept underlying these patents, is to provide a generally conical flow deflecting structure at the intersection of the grid strips, with the apex of the cone at the upstream end of the grid, such that the increasing cross-sectional area presented by the cone to the flow, produces lateral flow deflection. The flow through the grid adjacent a particular fuel rod, is deflected back toward the same fuel rod. Although the solid inverted cone such as described in U.S. Pat. No. 3,663,367 and its reissue, proved very effective in certain flow regimes, the grid pressure drop was in most cases excessive. The cone deflectors are more robust than the split vanes or tabs of the type described in U.S. Pat. No. 4,879,090, but the cones do not impart a swirling flow pattern to the coolant. A swirl pattern is desirable because it helps reduce the peak to average hot channel factor.